1. Field of the Invention
The present invention relates to an anchoring element for supporting a point mechanism, a method of applying an anchoring element within a bone, and a reconstructed joint.
A preferred embodiment of the invention will be described in detail in the following with reference to the reconstruction of a finger point. However, the invention is not limited thereto the invention can be used for other similar points, such as toe points, elbow points, and so on. The invention can also be used in reconstructions after amputation or other defects.
2. Description of the Related Art
Finger-point reconstructions, entailing prostheses, are primarily carried out on MCP joints which have become rheumatically changed. Prostheses known for this purpose include Swanson Silastic finger joints. Such joints consist of a material similar to silicon, are elongate in shape, and have ends with substantially circular cross sections. The ends can be fitted and anchored into the marrow cavity of each bone. The central portion of the prosthesis is elastically deformable and thus constitutes the actual joint mechanism.
The St. George prosthesis, a cemented prosthesis, is also known for finger-point reconstructions.
A problem with these conventional prostheses is that the anchoring element supporting the actual prosthesis mechanism loosens (mainly due to bone resorption) with undesired displacement of the prosthesis in the direction of load.
Attempts have therefore been made in recent Years to use titanium fixtures anchored in the marrow cavity of the bone with the object of becoming osseo-integrated as described by Hagert et al. "Metacarpophalangal Joint Replacement with Osseo-integrated Endoprostheses" in Scand. J. Plast. Reconstr. Surg. 20, pages 207-218, 1986. It is already known to permanently anchor oral and extraoral prostheses in bone tissue. This dental osseo-integration technique has been developed over the last 25 years by Professor Branemark and his colleagues, with excellent results in applying fixtures in the jawbone to hold teeth or arch attachments. However, the experiments performed by Hagert to apply this technique to the reconstruction of finger joints has not fulfilled expectations. The unacceptable results are evidently due to the entirely different conditions encountered when using this "dental technique" in the prosthetic reconstruction of finger-points. For example, in the known techniques, the fixture is anchored at right angles to the longitudinal axis of the bone. In the finger joint, the fixture is placed along the axis of the bone. Of course, this creates totally different loads and stresses on the anchoring elements.
Today, the main problem in orthopedic prosthesis surgery is still loosening of the bone anchoring unit. However, with a success rate for dental implants of more than 90% over a 20 year period, a number of other problems arise which, so far, have been unnecessary to take into account. One of the major problems is increased wear on the joint mechanism. A different type of prosthesis design from that used hitherto is required if the osseo-integration method is to be applied. To enable the joint mechanism to be replaced without disturbing the bone anchorage, the prosthesis system must be divided into components where the point-mechanism element can be separated from the actual bone-anchoring element. Further, if the two-stage method is to be used, it must be possible to connect the joint mechanism in the second stage if the patient, or at least the patient's reconstructed joint, is not to be kept immobilized. Two factors must therefore be taken into account: First, the joint mechanism is subject to shear and therefore must be replaceable. Second, to use the two-stage method, the joint mechanism must be replaceable.